A New Form of Chlorophyll?

Researchers may have found a new form of chlorophyll, the pigment that plants, algae and cyanobacteria use to obtain energy from light through photosynthesis. Preliminary findings published August 19 in Science suggest that the newly discovered molecule, dubbed chlorophyll f, has a distinct chemical composition when compared with the four known forms of chlorophyll and can absorb more near-infrared light than is typical for the photosynthetic pigments. Chlorophyll f, which was extracted from cultures of cyanobacteria and other oxygenic microorganisms, may allow certain photosynthetic life forms to harvest energy from wavelengths of light that many of their competitors cannot use.

"This is the most red-shifted chlorophyll we have found in nature," says Min Chen, a biologist at The University of Sydney in Australia and lead author of the study. "That means that organisms that have this chlorophyll inside can extend their photosynthetic range for maximum use of solar energy."

Some photosynthetic bacteria are known to use infrared light, but—in contrast to plants and cyanobacteria—these microorganisms do not produce oxygen. Instead, they rely on anoxygenic photosynthesis, which can function on the low-energy photons provided by infrared light. "Nobody thought that oxygen-generating organisms were capable of using infrared light, because the kind of photosynthesis that actually produces oxygen is thought to require a greater amount of photon energy from visible light," says Samuel Beale, a molecular biologist at Brown University whose work centers in part on chlorophylls. "I think what they found here is a new modification of chlorophyll that shows the flexibility of photosynthetic organisms to use whatever light is available."

Robert Blankenship, a photosynthesis expert at Washington University in St. Louis, agrees that the discovery is significant. "I think this is a very important new development and is the first new type of chlorophyll discovered in an oxygenic organism in sixty years," he wrote via e-mail.

Other researchers are more cautious about the findings. John Clark Lagarias, a molecular biologist at the University of California, Davis, points out that earlier research suggests some oxygen-producing cyanobacteria can harvest energy from near-infrared light using chlorophyll d—one of the four known varieties of chlorophyll, which also include chlorophylls a, b and c. But the new paper still interests Lagarias: "It's an exciting potential discovery, and if it's true it provides a second example of a red-shifted-chlorophyll-containing organism," he says. "We don't know for sure that it's used for photosynthesis, but we know it's absorbing light and it's likely to be involved in photosynthetic apparatus somehow. It could be a bona fide new form of chlorophyll that exists in something living."

In July 2008, Min's colleagues collected samples of stromatolites—structures formed from layers of cyanobacteria, calcium carbonate and sediments— and microbial mats from Hamelin Pool in Shark Bay, Western Australia, which is known to contain some of the most diverse and oldest stromatolites in the world. Cyanobacteria and other microorganisms build stromatolites in shallow water as they grow, gradually trapping and binding sediments into the small rock-like towers and mounds. Chen ground up the samples in a mortar and pestle and cultured the microorganisms in petri dishes under continuous illumination by near-infrared LEDs. Eventually, only microorganisms like cyanobacteria capable of photosynthesis using near-infrared light survived in the cultures.

Chen then used solvents to extract the living cells and pigments from the cultures and analyze their properties with a variety of laboratory techniques. The collective results suggested that the cyanobacteria contained a novel form of chlorophyll that can absorb near-infrared light up to 706 nanometers (nm) in vitro, with a fluorescence of 722 nm. Chen named the proposed variant chlorophyll f. A technique called high-performance liquid chromatography (HPLC), which separates molecules based on chemical properties (like whether they are hydrophobic or hydrophilic), confirmed that chlorophyll f is distinct from the four known varieties of chlorophyll. Nuclear magnetic resonance spectroscopy, which allows scientists to determine the arrangement of atoms within a molecular structure, reaffirmed the pigment's distinctiveness. And mass spectrometry, which determines the atomic mass of a molecule, revealed that chlorophyll f had an identical mass to chlorophyll b, which suggests they might be isomers of one another. "You can imagine an enzyme evolved that oxidizes the same precursor for chlorophyll b into this new form," Lagarias says.

Although Chen's results indicate the discovery of a novel light-absorbing molecule related to but distinct from known forms of chlorophyll, a few caveats complicate precise interpretation of her results. Firstly, the researchers had difficulty growing cultures of a single species, so it's unclear exactly which microorganism chlorophyll f comes from. Similarly, the researchers also struggled to grow cultures that yielded pure chlorophyll f untainted by other forms of chlorophyll. And a direct link between chlorophyll f and photosynthesis will require further research, which Chen says is now under way. "They haven't demonstrated that chlorophyll f is in the reaction center [the main site of photosynthesis]," Lagarias says. "But their results suggest the molecule is fairly abundant, so it probably plays some specialized role."

If cyanobacteria do in fact rely on chlorophyll f, then they might perform photosynthesis with light that is useless for most their neighbors—a significant advantage, especially in the dense and diverse communities of photosynthetic microorganisms that live within microbial mats and stromatolites and compete for energy from light. "In a microbial mat, infrared light not being absorbed by other organisms in the mat may be the only wavelengths of light available to you," says Lagarias says. "The implications are that this organism would occupy a critical niche and survive even though there are thousands of other organisms growing all around it."

Blankenship sees applications for biotechnology as well. "If this chlorophyll could be put into a plant and function properly, then it would be able to utilize some additional light energy that no plant now can use," he wrote via e-mail. "This has the potential to increase the efficiency of photosynthesis, as before energy storage can take place, the light has to be absorbed. Any wavelengths of light that are not absorbed are lost forever. A typical plant absorbs most of the sunlight in the visible region (400–700 nm) but very little beyond 700 nm [which marks the border between red and infrared light]. The visible region accounts for about half of the solar output energy. By pushing the absorption into longer wavelengths, an additional 10 percent% or so of the solar output is potentially useable."